High-Efficiency gas turbines add new flexibility

Siemens’ H-class gas turbine, the largest machine of its type in production, recently broke the 60% net thermal efficiency barrier in combined-cycle (CC) operation. Other gas turbine makers with efficient machines also are exploiting design and control advances in CC power plant applications—and in evolving hybrid CC plants that incorporate renewable energy.

Frank J. Bartos, PE

07/25/2011

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First it was high power output and unprecedented thermal efficiency of new industrial gas turbines that drew the attention of prospective customers. Now the latest generation of these machines demonstrates remarkable operating flexibility under varying loads—in combined-cycle gas turbine (CCGT) power plants—thereby widening their application. Besides the gas turbine, a modern CCGT power plant comprises a steam turbine, generator, heat recovery steam generator, myriad subsystems, and control systems to coordinate them. Precise control of the gas turbine is key to optimal plant operation.

Combined-cycle power plants are well suited for fast-response reserve power to balance large grid load fluctuations due to the rising share of electricity generated from renewable power sources. Short-time weather changes affecting wind power and diurnal interruption of solar power cause load fluctuations that make it difficult to keep the grid in balance. CCGT plants are here to help.

Breaking the 60% thermal efficiency barrier has been the goal of gas turbine manufacturers for some time. Major suppliers, such as Alstom, GE, Mitsubishi Heavy Industries, and Siemens, offer turbines “designed for 60+% efficiency in CC operation.” However, as far as it’s known, Siemens’ H-class SGT5-8000H machine was the first to set an independently verified record of 60.75% net thermal efficiency with 578 megawatt (MW) output at an actual CC power plant in May 2011 (see Ref. 1, online). The turbine went into commercial operation as part of the 8000H CC plant in late July 2011.

Efficiency milestone announcements by others included “gross efficiency,” expected numbers, or similar qualifiers. Unofficially, other turbine makers are at or above the 60% efficiency mark, which is good for customers and the industry.

Operational flexibility

While even tenths of a percent efficiency improvement translate into energy cost savings at some plant operating mode, ability to operate flexibly at different loads—not necessarily peak efficiency—also becomes important.

Siemens has consistently demonstrated the 8000H gas turbine’s flexibility for efficient use throughout base, intermediate, and peak-load ranges. Stable operation down to 100 MW has been shown in combined-cycle mode, which is under 20% of rated output—along with load ramp rates of up to 35 MW/min, explained Lothar Balling, head of Siemens Energy’s GT Power Plant Solutions.

“The requirement to balance grid power fluctuations cannot be met by existing energy storage facilities and only to a very limited extent by today’s nuclear or coal-fired power plants,” Balling said. “However, CCGT plants with rapid cycling capability can meet the challenge.”

Dr. Michael Suess, CEO of Siemens Energy Sector, added that, “Fast start-up and ramp-down times given for the 8000H turbine are verified and consistent.” He noted a further option available with this new breed of gas turbines. Adding a second gas turbine to the unit allows future plants to reach 1,100-1,200 MW output. “This way, a CCGT plant can match the output of large-scale power plants using other energy sources,” Suess stated.

As for the 60-Hz version H-class turbine, Siemens is confident about meeting the same efficiency numbers as the 50-Hz design. Scaled from the 50-Hz version and benefiting from experience gained during that lengthy development and validation, only limited work remains to prove out the 60-Hz model, according to Willibald Fischer, Siemens Energy’s program director for 8000H turbines. Orders for eight 60-Hz machines are on hand (Ref. 2, online), with some units being assembled for delivery in 2012.

Alstom Power likewise emphasizes operational flexibility of its CC power plant technology, recognizing that customers more often need to operate their gas turbines at part load. “Power markets are changing with the introduction of intermittent renewable power technologies like wind and solar,” said Mark Coxon, senior VP of Alstom’s Gas business. “Combined-cycle power plants are expected to operate over a wide range of operating regimes from base-load to daily start-and-stop, and quickly bring the missing (intermittent) power to the grid.”

Announced in early June 2011, Alstom’s “next generation” KA26 combined-cycle power plant integrates flexibility, using the extensively upgraded GT26 (50 Hz) gas turbine. New KA26 is “designed to enable 61+% efficiency in CC operation” and provide more than 500 MW output, according to Alstom.

“High part-load efficiency, not just base-load efficiency, becomes increasingly important as well as ramp-up speed of the power output,” continued Coxon. Part-load efficiency can often be even more important. He explained that efficiency of enhanced KA26 stays virtually constant down to 80% of full load, and it can operate down to 20% CC load.

This “parked mode” around 100 MW output provides a fast response standby to anticipate load ramp-up. “New KA26 can ramp up from low load to deliver more than 350 MW in less than 15 minutes. It can be started up in less than 30 minutes without a ‘lifetime penalty’ or the need for additional equipment,” Coxon added.

GT26 gas turbine incorporates two technologies—sequential combustion, a design feature invented by Alstom, and multiple variable compressor guide vanes—to optimize the difficult combination of high efficiency plus low plant turn-down with low emissions. In addition, GT26 has two online switchable control modes with maintenance implications. Performance optimized mode uses higher turbine firing and exhaust temperatures to maximize power output, while lifetime optimized mode lowers firing/exhaust temperatures, allowing up to 30% longer turbine inspection intervals, according to Alstom. Only a small performance reduction is said to result with the latter mode.

Going for 1,700 C

Meanwhile in February 2011, Mitsubishi Heavy Industries Ltd. (MHI) started test operation of a J-series (M501J) gas turbine at the company’s CC power plant for verification testing. Said to offer “the world's largest power generation capacity,” the 60-Hz turbine has power output of 460 MW in combined-cycle operation. Under commercialization for more than two years, J-series gas turbine achieved gross thermal efficiency exceeding 60% and 1,600 C inlet temperature in test operations, according to MHI.

That temperature is 100 degrees higher than the company’s previous G-series turbine, a workhorse machine with more than 700,000 cumulative operating hours. Looking ahead, industrial gas turbines with still higher inlet temperatures are under development at Mitsubishi. MHI is part of an ongoing Japanese national project with the objective to develop “core technologies” needed for a new turbine class capable of 1,700 C inlet temperature. New technologies derived from the project are being adopted in J-series machines, according to the company.

The objective of going to higher turbine inlet temperatures is clear. It allows thermal efficiency improvement, but it also subjects turbine components to higher thermal stresses and complicates material choices and overall design. Consequently, other major turbine makers design for lower inlet temperatures, generally under 1,500 C. First shipments of MHI’s J-series turbine are expected in 2011 to a Japanese power company. Development of a 50-Hz gas turbine (M701J) is also underway, with first shipments slated for 2014, says the company.

Still higher efficiency?

Gas turbine manufacturers will undoubtedly continue the quest for higher thermal efficiency, but it’s a case of “low-hanging fruit already picked.” Further gains will likely be smaller and require difficult material and design choices.

Siemens Energy’s Fischer said, “We’re getting close to efficiency ceilings, but some possibilities remain.” Without revealing to competitors what steps might be taken, Fischer mentioned that more than 61.5% net efficiency is likely for Siemens turbines in the 2015 time frame.

As for the 1,700 C inlet temperature turbine project at Mitsubishi Heavy Industries, a 62% to 65% efficiency range in CC operation is an estimate. Larger gains can come from hybrid CC power plants (see below) because they change the efficiency equation. GE Energy’s 50-Hz 9FB, another upgraded gas turbine with long service history, is featured in the trademarked FlexEfficiency 50 combined-cycle power plant recently announced by the company. GE puts the CCGT plant’s “expected base-load efficiency at greater than 61%” and rated output at 510 MW. Part-load efficiency values are also said to be favorable, for example, 60% efficiency at 40% of rated load. Reportedly, FlexEfficiency can reach rated output in 1 hr from cold start (28 min from hot start) and will provide 50 MW/min ramp rate.

Current focus on gas turbines at GE Energy is on F-class machines, according to a company spokesman. Earlier, GE developed H-class turbines now running at CC power plants in the U.K., Japan, and the U.S. (see Ref. 3, online).

Gas + wind + solar synergy

Much has been said about applying CCGT power plants to balance and control electricity flow on the grid, filling in for intermittent power generated from renewable energy sources, such as wind and solar. However, CC can more than balance grid loads. Modern CCGT technology can provide the basis of various hybrid power plant designs that integrate renewable power, thereby further raising thermal efficiency of the overall plant.

Such a development was announced by GE in early June 2011 when its FlexEfficiency CCGT technology was selected by Turkish project developer MetCap Energy Investments for an integrated renewables combined-cycle (IRCC) power plant with wind and solar power to be built in Turkey. FlexEfficiency 50 will be used to integrate all elements of a CC plant plus 22 MW of GE Energy wind turbines and 50 MW of eSolar concentrated solar thermal tower technology.

Located in Karaman, Turkey, the IRCC plant, which GE claims to be the world's first, will be rated at 530 MW output and achieve 69% efficiency at site conditions, according to GE. Latest version of the 9FB turbine is at the core of the plant’s CC section. A GE Mark VIe plant control system will integrate and control all the diverse plant elements. Commercial operation of the IRCC power plant is scheduled for 2015.

eSolar, an advanced concentrated solar thermal tower technology provider, will supply that part of the IRCC plant. GE will incorporate eSolar technology and software into its IRCC and stand-alone solar thermal power plants, under an investment and licensing agreement.

In simplest terms, eSolar’s technology uses small, flat mirrors, sensors, and motion controls to accurately track the sun, concentrating solar heat to a tower-mounted receiver where water is turned into steam. In the IRCC plant, solar-generated steam feeds the steam turbine of the CC plant to generate extra power without using more natural gas.

Hybrid CCGT technology will require thorough site-specific evaluation for suitability and economic sense. However, further development of efficient gas turbines can stimulate growth of CC power plants, whether conventional or an evolving hybrid variety.

Frank J. Bartos, PE, is a Control Engineering contributing content specialist. Reach him at braunbart@sbcglobal.net.

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